Page 229 - Complete Wireless Design

Oscillator Design

228 Chapter Four

1. Choose a proper high-frequency transistor with an f that is much higher
T
than the oscillation frequency (5 times higher is a good choice).
2. Bias the active device Class A by the following procedure:
a. Choose the supply voltage. Select a Q point for the transistor that is con-
sistent with the available S-parameter file for I and V . Example: (I
C C C
10 mA; V 6 V; V 12 V. Find transistor’s typical , such as 50.
C CC
b. Calculate I I /
B C
V 0.7
C
c. Calculate R
B
I
C
V V
CC
C
d. Calculate R
C
I I
B C
3. Calculate the values for the LC resonator and other components by:
190 1 1
L C C
2 f 1 48 f 2 48 f
1 2500
C C 1 ohm (X ) R
3 C C f
300 f (0.025/I )
C
(R should be tweaked in the preliminary open-loop S-parameter analysis
f
until both the input and output are close to 50 ohms on the Smith chart.)
4. C ≈ 50 to 200 ohms (X ) for a 50-ohm load. Find the necessary value
COUP C
of C by simulating the oscillator into a 50-ohm load, and use the lowest
COUP
C reactance value that will still allow the oscillator to maintain a
COUP
decent gain margin ( 5). If a high input impedance buffer amplifier follows
C , then C C (however, the phase noise will go up).
COUP COUP C
5. Simulate and optimize as explained in Section 4.1, “Oscillator Simulation.”
Notes. Increasing C and C , as well as L, while decreasing C , will increase the
1 2 3
loaded Q of the oscillator (high loaded Q reduces phase noise and frequency drift, and
reduces temperature effects).
While this oscillator is capable of operation at up to 500 MHz, it does have a lower
frequency limit of about 25 MHz, at which point it is advisable to work with a Colpitts
design.

MMIC LC oscillator (Fig. 4.16). This is an oscillator that is capable of up to 1
GHz frequency operation. It is much simpler to design, but will cost more,
than the LC BJT oscillator above. The MMIC oscillator is only used when
higher-frequency operation is required.

1. A V should be chosen that will allow at least 2 V (preferably 4 V) to be
cc
dropped across R for stability. (If R does not reach 500 ohms, employ
BIAS BIAS
an RFC for a combined impedance of 500 ohms for both R and the RFC):
BIAS
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